1. Field of the Invention
The present invention relates to a semiconductor device, more particularly to a semiconductor device including a so-called trench gate type MOS transistor.
2. Description of the Related Art
In recent years, demands of power MOS transistors are increasing rapidly, not only in a market of switching power supplies with large current and high breakdown voltage (for example, a breakdown voltage of 600V), but also in a market of energy-saving switching devices used in a mobile communication device including a notebook PC (for example, a breakdown voltage of 30V).
A power MOS transistor used in such an energy-saving switching device market is required to reduce its driving voltage so that it can be driven directly by a voltage from a battery. It is also required to reduce its ON-resistance, and its gate-drain capacitance for reduction of switching loss, and so forth.
On the other hand, an ON-resistance of a MOS transistor mainly consists of a channel resistance and drift resistance. A structure called super-junction structure is known as a structure of a MOS transistor for reducing a drift resistance.
A super-junction structure forms a drift layer as a structure in which longwise strip-shaped p-type pillar layers and n-type pillar layers are embedded in turn in the transverse direction (for example, see JP 2003-273355 A). When a charge quantity (an impurity quantity) in a p-type pillar layer is equal to that in an n-type pillar layer in a super-junction structure, a pseudo non-doped layer can be formed therein. Thereby, a low ON-resistance beyond a material limit is realized by flowing a current in a high-doped n-type pillar layer, while keeping a required breakdown voltage.
Even in a power MOS transistor having a breakdown voltage of about 30V suitable for an energy-saving switching element, it is possible to form a MOS transistor with a low ON resistance by forming a drift layer of super-junction structure.
However, in a power MOS transistor having a low breakdown voltage of about 30V used for an energy-saving switching element, a pitch of a super-junction structure needs to be shortened to about 1 micrometer, which is shorter than a power MOS transistor of a large current and a high breakdown voltage. Such a narrow-pitched super-junction structure must be formed in the last step in a process including a MOSFET-forming process, in order to prevent thermal diffusion. Thus, it is difficult to form such a narrow-pitched super-junction structure not only in an element region but also in an end region surrounding the element region.
For this reason, in a power MOS transistor with a breakdown voltage of about 30V used for an energy-saving switching element, concentration of electric field in an end region could not be eased enough. Therefore, a breakdown voltage in an end region cannot be high enough compared to that in an element region. This is recognized as a problem.